The SIAI-Marchetti S.211 (later Aermacchi S-211) is a turbofan-powered military trainer aircraft designed and originally marketed by SIAI-Marchetti. Some 60 aircraft have been sold to air forces around the world. Aermacchi bought the production rights in 1997. The redesigned M-311 (now M-345) is currently under development by Alenia Aermacchi.

SIAI-Marchetti began work on the aircraft in 1976 as a private venture basic trainer, hoping to offer it to the company's existing customer base of small air forces operating their piston-engined SF.260. Formally announced in Paris the following year, interest was strong enough to justify the construction of two prototypes, the first of which flew on 10 April 1981. The Singapore Air Force placed the first order for ten aircraft in 1983.

The S-211 is a small tandem two-seat shoulder-wing monoplane with a retractable tricycle landing gear and powered by a single Pratt & Whitney Canada JT15D-4C turbofan. Also, the aircraft was designed for a secondary close air support capability with four underwing hardpoints for the carriage of weapons and other external stores.

The S-211 design evolved into the Aermacchi M-345. The airframe has been strengthened, and a glass cockpit and modern avionics have been added. As a basic trainer, the aircraft is designed to complement the Aermacchi M-346 advanced trainer. The aircraft was designed with a secondary close air support capability with five underwing hardpoints.
The prototype aircraft first flew in June 2005.